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Cracked rotor bar research project (2300HP 8 pole motor)

Cracked rotor bar research project (2300HP 8 pole motor). Herman vd Merwe (Engineering Manager). Revision: November 2005. Cracked rotor bar research project (2300HP 8 pole motor). Rationale :. Catastrophic failure on electric motors, e.g. ID fan motor (Eskom, 11kV, 7250kW).

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Cracked rotor bar research project (2300HP 8 pole motor)

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  1. Cracked rotor bar research project (2300HP 8 pole motor) Herman vd Merwe (Engineering Manager) Revision: November 2005

  2. Cracked rotor bar research project (2300HP 8 pole motor) Rationale: Catastrophic failure on electric motors, e.g. ID fan motor (Eskom, 11kV, 7250kW) Cause of failure (generic sequence of events): • Bar of copper type rotor cage breaks __ current continues to flow in the bar • Close to break location, current flows through core laminations to adjacent unaffected bars • ‘Inter bar’ currents cause electrical arcing between core laminations and the broken bar • Localized heating could be so severe that the bridge portion of the laminations melts away • Broken bar then lifts out of the core slot under influence of centrifugal forces • Lifted bar invariably starts to rub against the overhang of the stator winding • This rub leads to an inter turn fault on the stator winding and thereby an earth fault

  3. Cracked rotor bar research project (2300HP 8 pole motor) Findings: Cracked rotor bar lifted out of the core slot (close-up & general view)

  4. Cracked rotor bar research project (2300HP 8 pole motor) Findings (continued – 1): Damage to stator winding overhang (close-up of failed coil & other damaged coils)

  5. Cracked rotor bar research project (2300HP 8 pole motor) Findings (continued – 2): Bridge of rotor laminations destroyed Overheated bridge of rotor laminations

  6. Cracked rotor bar research project (2300HP 8 pole motor) Findings (continued – 3): Electrical arcing damage on broken rotor bar (bottom of slot side)

  7. Research project, facilitated by the load test plant of M&C and involving Eskom and various industry and university based specialists Cracked rotor bar research project (2300HP 8 pole motor)

  8. Cracked rotor bar research project (2300HP 8 pole motor) Basic research methodology: • Use a wide range of test techniques (existing technologies & experimental / new) and measuring instruments, operated by a number of test personnel from different companies Tests under load – Vibration spectral and time waveform analyses Stator current spectral analyses Stray flux measurement Monitoring of shaft earthing brush current Other tests: Assembled off-line tests (inductance profile) Disassembled off-line tests (bar continuity and ‘growler’) • All tests are conducted for a number of progressive test conditions ( 1. “as is”, 2. half a broken bar & 3. one broken bar; tests with two broken bars required?…) • Tests under load are taken at a number of different load points

  9. Cracked rotor bar research project (2300HP 8 pole motor) Artificially introduced progressive test conditions: One rotor bar half broken One rotor bar fully broken

  10. Cracked rotor bar research project (2300HP 8 pole motor) Test results, analyses and comparison of techniques *** TO FOLLOW… ***

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